Process and apparatus for manufacturing set cellular cement

ABSTRACT

A process for manufacturing set cellular cement, including the steps of: (i) mixing cementitious material, water, foaming agent and optionally additives into a free flowing slurry having a slump of at least 100 mm; subsequently (ii) injecting and distributing air into the slurry of step (i) to form a cellular slurry; subsequently (iii) casting the cellular slurry of step (ii); and finally allowing the cellular slurry to set. And, an apparatus for carrying out the process.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority of EP 04290494.6, filed on Feb.24, 2004, the subject matter of which is incorporated herein byreference.

FIELD OF THE INVENTION

The instant invention relates to a process and an apparatus formanufacturing cellular cementitious slurry and the set material obtainedtherefrom. The invention also relates to processes and apparatusesincorporating the instant process. The instant invention refers in thefirst place to the manufacture of plasterboard and more specifically tothe manufacture of a plasterboard core on continuous plasterboard lines.

DESCRIPTION OF RELATED ART

Cementitious materials are known for many years. Examples ofcementitious material can be gypsum (which is available in many forms),Portland cement, sorrel cement, slag cement, fly ash cement, calciumalumina cement, and the like.

Plasterboard consists, grossly speaking, of two sheets of a materialhaving a certain tensile strength, like paper, covering a core,essentially of cement generally gypsum, with a certain compressivestrength. The flexural strength of composite material depends upon thecombined strengths of the components.

One element influencing the strength of the core is the water/plasterratio used for the preparation. A rule of thumb is that the compressionstrength of a cast gypsum body increases with the square of its apparentdensity. In the range of application the density increases nearlylinearly with the inverse of the W/P ratio. Thus, a low W/P ratio istraditionally considered as favorable.

The core of plaster wallboard is usually lightened by incorporation ofair into the core preparation. The air in the core appears in the formof bubbles. It has been found that the size and the distribution of thebubbles have influence on the mechanical properties of the core andtherefore of the board. A broad size distribution of bubble diameter andevenly scattered over the bulk are favorable. A layer of dense material,without or with fewer voids, near an optional covering is favorable. Inthat respect, one may revert to U.S. Pat. Nos. 5,085,929 and 5,116,671to Bruce, hereby incorporated by reference.

The air is usually introduced into the plaster slurry in the form ofprefabricated foam. In the normal foam generation systems, a quantity offoam-generating surfactant is diluted with water and then combined withcompressed air. Foam is generated using various devices and processes.This foam is injected into the mixer, usually directly in the mixer. Themixer, which is usually a high shear mixer, assures the foam iscompletely combined with the plaster slurry but at the cost of a hugereduction in foam efficiency. The volume of foam added to the slurry istypically 3 times the volume actually combined in the board. Therefore,in accordance with the classical prior art, part of the gauging waterfor the plaster is added with the foam. More water in the foam raisesthe density of the foam and allows more uniform mixing with the plasterslurry, which is of higher density than the foam. However, thisadditional water reduces the final strength of the gypsum matrix byunnecessarily increasing the space between the gypsum crystals and,thus, forming a weaker structure.

U.S. Pat. No. 5,575,844 to Bradshaw discloses a secondary mixer (mountedin the same casing), in which the foam is introduced, while water andplaster are introduced in the primary mixer. The first mixer is forplaster and water while the second is for foam addition, where the shearis lower.

U.S. Pat. No. 5,714,032 to Ainsley discloses a two-chamber mixer,comprising a first, high-shear, chamber and a second, low-shear, chamberin which the foam is introduced.

U.S. Pat. No. 5,683,635 to Sucech discloses a process in which the foamis inserted into the slurry at a point where it is less agitated thanduring the creation of the slurry in the first mixer, whereby the foamis less agitated than if inserted in the pin mixer itself.

While these documents provide processes with lower foam consumption,additional water is still combined with the foam to the detriment of thefinal gypsum core properties.

Further, these documents disclose processes that still provide the usualpore volume with no control over the size and distribution of thebubbles.

Direct air injection during the creation of the cementitious slurry isalso known.

U.S. Pat. No. 6,443,258 to Putt discloses a process for making soundabsorbing panels in which plaster, fibers, water and foaming agent aremixed and simultaneously aerated using a mixing device similar to akitchen aide mixer, orbiting and rotating mixing device. Air isentrapped, from the ambient, in the slurry, where the entrapment resultsfrom the combination of a dry mixture of plaster, (and optionaladditives) and of an aqueous mixture of water and surfactant.

DE-A-2,117,000 to Anton discloses a mixer for producing wall-finishingmortar. The apparatus can be worked according to two embodiments. In thefirst one, air is forced in a flow of gauging water, where said waterhas been through a cartridge filled with a surfactant. What isintroduced in the mortar mixer is actually foam (pressurized foam). Inthe second embodiment, no surfactant is mentioned. Air is introduced inthe slurry through a porous fritted glass member, at a level of themixing screw of the unique mixer that is used. The type of mixer used inthis document is not suited for the production of boards or panels,since the slurry that is produced is of high viscosity so as to adhereto the wall, making this slurry completely unsuited for the conventionalproduction of boards or panels. Last, this type of mixer presents thedrawback of a lot of air loss. This design presents the fatal flaw ofbeing a pump of constant volume and with no control of share of airentering the pump. This causes a variation in the water to plasterratio.

U.S. Pat. No. 6,376,558 to Bahner discloses a conventional mixer inwhich air is introduced under pressure through a porous fritted glasssituated in the walls of the rotating mixer. In this unique mixer, theslurry is generated in a one-step process, since all components of theslurry are introduced at the same time in the mixing chamber. Thisdevice can entrain air carried into the mixer by the plaster.Furthermore, the condition for distributing the air into the slurry willvary according to the composition of the slurry, the flow rate throughthe mixer, and will be more variable as the mixer is worn by the slurry.

U.S. Pat. No. 2,097,088 to Mills discloses a conventional mixer forplasterboard in which air is introduced under pressure through apertureslocated in the bottom part of the mixer. Said mixer is said to be suitedfor mixing plaster and fibers. This document did not recognize the issueof the foaming agent and the foam stability, since foaming agents werenot used at that time. In this unique mixer, the slurry is generated ina one-step process, since all components of the slurry are introduced atthe same time in the mixing chamber. As in the Bahner reference thisdevice can entrain uncontrolled air carried into the mixer by theplaster.

U.S. Pat. No. 5,250,578 to Cornwell discloses a foamed cellularcementitious composition useful for sound-absorbing. The components,inter alia gypsum, water, foaming agent and film-forming agent, anaggregate, optionally fibers, and air can be combined in a slurrypreferably by the classical foam introduction into the slurry. The aircan also be introduced by mechanical agitation.

U.S. Pat. No. 1,687,067 to Hinton discloses a continuous process formaking cellular cementitious material, in which a high-viscosity pulp(containing a so-called frothing flotation reagent or flotation oil) isagitated in a reactor, where air is bubbled from the bottom of thereactor and the foamed cementitious pulp is added well above the diskoverflows from said reactor at a nearly equivalent level. The bubblesthus-formed are said to be “fine bubbles”, due to the use of a rapidlyrotating perforated disc or other means placed immediately above the airdistributing plate. The air, in this method, that is entrapped would bepoorly mixed into the slurry, especially for quick-setting cement. Themixer as described is not suitable for rapid setting cements because itpermits long residence times due to the proportion of length to diameterand the vertical orientation. There is no mention of the products thatcould be manufactured using said process.

U.S. Pat. No. 1,660,402 to Thompson discloses a process for producingcellular cementitious material. In a first step a slurry (e.g. gypsumand water) is first produced, in a vortex mixer which does not allow theaddition of foaming agent into the gauging water. This slurry is thenintroduced into an air-mixing chamber, where air bubbles are created.The air is agitated into the slurry without control over the quantity orform of the voids in the slurry. Colloidized water (e.g. with saponinwhich is the sole agent referred to in the text that could function as afoaming agent) is then introduced, where this liquid will act as afoaming agent. Hence, this process relies on the addition of the foamerafter the air bubbles have been created in the slurry, where the foamerintroduced further adds water to the initial amount of water, andwithout control of the form of the bubbles in the hardened mass. Thediluted foaming agent is introduced into the second mixer, where thisadditional water has the same effect as the water added in theprefabricated foam of later designs.

U.S. Pat. No. 5,013,157 to Mills discloses process and an apparatus formanufacturing aerated cementitious slurry. Dry cementitious componentsare mixed in a screw mixer; the blend is discharged into a hopper, wheresaid hopper is also connected to a water feeding device at its bottompart while being free at its upper part. The wet slurry then enters afurther screw pump, the rotation of which creating air suction andconsequently air entrainment into the wet slurry (since the ratedcapacity of the pump is greater than the rate at which the wet slurryare fed to the mixing inlet). Aerated slurry is thus formed.

U.S. Pat. No. 5,660,465 to Mason discloses a process and apparatussimilar to the one disclosed in U.S. Pat. No. 5,013,157 above. In Mason,the water is fed at the same time to the first screw pump, so that aslurry exits said first pump. The slurry is then similarly fed from achute into a hopper, where said hopper is connected to a positivedisplacement progressive cavity slurry pump. By adjusting the rotationspeed, the ratio of slurry to entrained air can be modified.

In the above documents to Mills and Mason, whenever a pump is used forentraining air, this does not result in favorable results since thosepumps mentioned are not mixers and do not blend correctly. At best thepumps can be qualified as kneading machines, which cannot create foams.

None of the above documents discloses air injection matured into areliable, industrial process used for the manufacture of plasterboard orpanel.

There is thus still a need to provide further mixing apparatus andprocess that would permit control of the bubble structure with the goalof producing high quality foamed or cellular slurry.

None of the above documents teaches or discloses the instant invention.

SUMMARY

The invention thus provides a continuous process for manufacturing setcellular cement, comprising the steps of: (i) mixing cementitiousmaterial, water, foaming agent and optionally additives into a freeflowing slurry having a slump of at least 100 mm; subsequently (ii)injecting and distributing air into the slurry of step (i) to form acellular slurry; subsequently (iii) casting said cellular slurry of step(ii); and finally (iv) allowing said cellular slurry to set.

A mixing device for manufacturing a cellular cement slurry, comprising:(i) at least one first mixing device comprises a cement inlet and awater and foaming agent inlet, said first mixer being a high-shear mixeroperated under conditions to prepare a fluid slurry; and (ii) at leastone-second mixing device comprising air injection means, said secondmixer being operated under controlled-shear conditions.

A mixing device for manufacturing a cellular cement slurry, comprising:(i) at least one first mixing device comprising a cement inlet and awater inlet, said first mixer being a high-shear mixer operated underconditions to prepare a fluid slurry; (ii) at least one further firstmixing device comprising a fluid slurry inlet and a foaming agent inlet;and (iii) at least one second mixing device comprising air injectionmeans, said second mixer being operated under controlled-shearconditions.

An apparatus for manufacturing a set foamed cement body comprises (a) atleast one mixer according to the invention, (b) means for casting acellular slurry and (c) means for moving a facer.

A preferred embodiment is based on the use of two mixing steps that arecarried out separately: the first one mixes the cementitious material,water and foamer. The second mixing or blending step is carried out toincorporate air. These mixing steps are preferably carried out atdifferent conditions, the first being under high-shear in order tocreate a homogeneous slurry while the second is under controlled shearand flow path in order to create a desired foam structure. Controlledshear conditions are those conditions which the skilled man may selectdepending on the slurry, the rate of injection of air, and the finaldesired cellular or void structure. For example, depending on the slumpof the slurry, the controlled-shear conditions will be either towardslow-shear or towards higher-shear (but still substantially lower thanthe high-shear conditions of the first mixer) if one is seeking ratherlarge or rather fine bubbles. The type of second mixer of blender willalso have influence, as well as the type of foamer, additives, etc. Theskilled man will know by routine tests how to determine and apply thecontrolled-shear conditions in order to obtain the desired voidstructure.

The basic idea of the preferred embodiment is to use the slurry as theliquid used to create a foamed slurry. The foaming then happensessentially without the addition of water which necessarily comes withprefabricated foam since only air is added in a second step. This doesnot exclude the optional addition of liquid additives, which wouldpreferably not exceed two percent by weight of the total slurry. Thisalso does not exclude using prefabricated foam in the first step. Thisdoes also not exclude staged addition of the components, where gypsum,water and optionally additives would be added in the first mixer, whilethe foaming agent would be added at about the exit of the first mixer,prior to the feeding to the second mixer providing air blending.

By applying the preferred embodiment, the dimension and distribution ofthe foam bubbles can be controlled by the blending conditions and flowpath. The result is a foamed body, which can be optimized to form eithera stronger or lighter body or to use less foaming agent and less waterthan the existing process to produce normal weight boards.

The process of the embodiment allows an optimization of high qualitycementitious slurry in the first stage, and foamed slurry withcontrolled (even bimodal) bubble size and distribution. A bi-modaldistribution might be created by separating the discharge from theplaster mixer into two different air blenders. The different streamscould be gently recombined into a true bimodal distribution.

High shear mixers should preferably have relatively small internal spacewith a low residence time, and the high shear avoids clogging in themixer. The controlled shear mixer with plaster should also preferablyfulfill certain criteria in order to prevent clogging or scaling in themixer. One preferred feature is the design of an internal cavity whichwill avoid recirculation of the slurry before discharge. Other featuresknown in the art can also be applied (keeping inlet open with thematerial moving to the discharge; special liners and/or flexible walls;heating parts where phase bounders occur, etc.). Preferably, the secondblender will generate a rather sharp residence time distribution.

Another element of the present embodiment is the control of the airincorporated in the slurry by assuring the “net” air void incorporatedin the slurry, since all air introduced into the slurry in the secondmixer will be incorporated in the final cementitious product.

The process of the invention also provides plasterboards and panels withenhanced properties.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is disclosed with reference to the following drawings.

FIG. 1 is a schematic representation of the invention;

FIG. 2 represents a first embodiment of a high-shear mixer of theinvention;

FIG. 3 represents a second embodiment of a high-shear mixer of theinvention;

FIGS. 4, 4 a and 4 b represent an embodiment of a controlled shear mixerof the invention;

FIG. 5 represents a second embodiment of a controlled shear mixer of theinvention;

FIGS. 6 and 6 a represent a third embodiment of a controlled shear mixerof the invention;

FIG. 7 represents a fourth embodiment of a controlled shear mixer of theinvention;

FIG. 8 represents a fourth embodiment of a controlled shear mixer of theinvention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The invention is disclosed in more details below, where the embodimentsare not given in a way to limit the practice of this invention.

With reference to FIG. 1, the process of the disclosed embodimentcomprises dry component(s) metering means 1 and liquid component(s)metering means 2, a multi-stage mixer 3, and a forming device 4, saidforming device being a classical one. The multi-stage mixer 3 comprisesa primary mixer 5, being preferably a high shear mixer, and a secondarymixer 6, being preferably a controlled shear mixer.

The foaming agent is metered in the first mixer together with the othervarious components (dry and liquid). Air metering means 7 are providedin the secondary mixer 6. This air metering means 7 will deliver therequired amount of air needed to produce the cellular slurry. Thecellular slurry is then delivered to a classical forming device 4,optionally by a spreading device 6 a.

FIG. 2 is a schematic representation of one embodiment of the first,primary mixer used in the invention. A similar mixer is disclosed inDE-A-3,138,808, incorporated herein by reference. The mixer 5 comprisesdry and liquid metering means 8, feeding into one device. Said deviceuses an overflow of liquid into a funnel 12. Dry materials (cementitiousmaterial and dry additives if any) are metered and combined into afeeding device which discharges into the funnel 12. Liquid additives aremetered into the liquid phase via the pipe 13 a. The pipe goestangentially into the vessel 13 letting rotate the liquid which, then,flows evenly over the rim of the funnel 12. A scraper 9 is placed infunnel 12. The scraper 9 and the open screw 10 are driven by a motorassuring that no dry material sticks on the walls. Pipe 15 will thenfeed a rotating high-shear mixer 5 a. Any type of known high-shear mixercan be used. Examples are pin mixer, Gorator®, rotor/stator mixer anddisc mixer.

One preferred mixer is an inclined disc mixer. The inclined arrangementwith the discharge 19 at the highest point avoids the capture of ambientair into the slurry. The pipe 15 is connected to the mixer through inlet16. A motor-driven disc 17 rotates at high speed in housing 18. The disksits eccentrically in the case, touching the wall at the dischargelocation in order to avoid the feed taking a “short cut” to thedischarge without passing through the mixer or eventually residing inthe mixer. The cementitious slurry then leaves mixer 5 a through thedischarge outlet 19. Control can be achieved by acting on liquid flowrate in pipe 13 a and/or on the flow rate of the cementitious materialby screws 10 and 11. Typical dimension for a 20 m³/h flow rate is adiameter of the disc of about 80 cm.

FIG. 3 is a schematic representation of a variant of the embodiment ofFIG. 2. In FIG. 3, one will recognize the funnel 12, recipient 13, andassociated pipe 13 a, feeding screw 11, screw 10, and pipe 15. In thisembodiment, the scraper-screw device 9 is mounted to rotate in funnel12. Its driving force is delivered to 9 by a motor (M1). A further axis20 will rotate inside the scraper 9 and screw 10, where an extra motor,M2, will drive said axis 20. Said axis 20 will extend further downstreamwith respect to screw 10. Axis 20 is equipped at its bottom with aturbine 21. Said turbine can be any turbine known in the art, such as animpeller turbine, an indented high-speed disc, etc. Said turbine 21rotates at high speed, thereby creating high-shear in the medium. Thecementitious slurry will then be discharged through pipe 22, which mayoptionally be equipped with a flow control valve 23. A sensor 24 fordetecting the mixture level in the space 14 or at any other locationalong pipe 15 may also be provided (said sensor might also be providedin the embodiment of FIG. 2). Such a sensor allows for a better control,in which sensor 24 may command control valve 23 and/or liquid flow ratein pipe 13 a and/or flow rate of cementitious material by screws 10 and11. Typical dimensions are a diameter of about 20 cm and a length of themixing zone of once to twice the diameter.

The cementitious slurry leaving the high-shear mixers, such as thoseembodied in FIGS. 2 and 3, but not limited to them, is then sent to acontrolled-shear mixer in which the cementitious slurry is blended withair so as to create cellular cementitious slurry. Many controlled-shearmixers can be used to that end.

One blender can comprise a porous fritted plate made from glass, metal,synthetics or ceramics. Such porous fritted plate can have pore sizes inthe order of ten microns, for a thickness of about a few millimeters.The air injection devices as disclosed in DE-A-2,117,000 and U.S. Pat.No. 6,376,558 are appropriate. Notably, a stirrer in a housing wherepart of the wall comprises a porous fritted glass is suitable. Manystirrers (screw, wire stirrer, etc.) are appropriate. Alternatively, aircan be injected using a dip-leg or any other suitable air-injectiondevice. Air can also be introduced by a multitude of holes, or thoughscreens, or preferably through nozzles injecting the air.

FIG. 4 discloses a first example of a secondary blender. It consistsprincipally of a horizontal tube 30 with a rotating stirrer shaft 31along its long axis. The stirrer is driven by a variable drive 32. Theorientation of the feed of the primary slurry is not an essentialelement. However a preferred embodiment is tangentially from the top ofthe horizontal tube. Different emplacements of the feeder orifice 33 arepossible in order to adapt the average residence time of the slurry inthe second mixer. For the same purpose a separation disk 34 can modifythe active volume of the tube according to slurry requirements. Thestirrer can be of the “squirrel cage type” 30 a, as shown in FIG. 4 a.As shown in FIG. 4 b, the stirrer can comprise other means foragitation, for example tended wires 30 b and/or screw-like springs 30 c.The aerated slurry leaves the blender through an outlet 35 opposite tothe receiving end. The orientation of the outlet 35 is preferably upperside in order to keep the blender full. Air can be injected by means offritted bodies 36 arranged along the bottom side of the tube. The air isunder pressure and metered by valves 37 and flow meters 38. A variant,not shown here, is with the case being conical shaped with the largerdiameter at the discharge end. In this case the bottom side of the casemight be horizontal so that the agitator axis points upward toward thedischarge.

FIG. 5 discloses a different type of secondary blender. It consistsmostly of a vertical cylindrical mixing vessel 40, a bottom with slurryfeed which can be central, as shown in 41, or lateral and frittedelements 42 for the air injection. A stirrer with (optionally) variabledrive 44 and agitating elements 45 creates the foamed product. A valve46 and a flow meter 47 control the airflow. The discharge is on theupper part, the slurry exiting as an overflow. An inlet 49 for meteredliquid additives is optional.

The secondary blender displayed in FIG. 6 is a vertical blender as well.Air is introduced by means of one or several nozzles 50 which may bemounted to inject air tangentially around the circumference of themixer. The feed of the primary slurry 51 is tangential with respect tothe lower end of the mixing vessel. The feed of slurry and air convergein a venturi nozzle creating a pre-blend. The outlet 52 is on the upperpart, as in the previous figure. The stirrer 53 is equipped with amultitude of, preferably, elastic wires 54 made out of metal or plastic.An inlet 55 for metered liquid additives is optional. FIG. 6 a is a topview of this embodiment.

In a variant of the blenders of FIGS. 5 and 6, not shown here, theblender vessel is closed on the top end, but leaving a certain spaceover the outlet. At the upper side of the cover is a level sensorcapturing the level of the slurry and a pipe, equipped with a manometer,a pressure control valve and a flow meter. The pressure control valve isguided by the level sensor in such a way that the level of the slurryremains constant with regard to the discharge. The manometer allows tomonitor whether a resistance is built up in the discharge/distributingsystem. The flow meter in collaboration with the flow meter 47 allowsmonitoring the fraction of air entrapped. It would also allow theoverflow discharge to work against a resistance, for example, adistributing device.

FIG. 7 discloses an embodiment of the invention, which combines the stepof injecting air and spreading the foamed slurry on a facing material.The cementitious slurry is discharged from the high-shear mixer througha discharge pipe 60 (which may be connected to the devices embodied inFIGS. 2 and 3 or any other suitable primary mixer). As known in the art,non-foamed cementitious slurry will spread without alteration incontrast with foamed slurry which can segregate when large bubbles arepresent or can coalesce over the length of the displacement. Hence, inthe instant embodiment, cementitious slurry spreads over a plate 61. Thecementitious slurry will then flow from plate 61 into a horizontalblender 62 in some respect similar in its concept to the one designed inFIG. 4 but working in cross-flow rather than along its axis. Thisblender 62 which acts as a stirring and air injection device, whichcomprises a rectangular container with vertical rear 63 and front 67walls and a half-round bottom. The lower part of the rounded part,referenced 64 comprises porous fritted elements 65 that may extend overabout 10 to 50% of the circumference. Air is injected through saidfritted elements into the cementitious slurry to form cellular slurry. Arotating stirrer 66, fitting in the rounded part, will ensure theblending of air with the slurry. The stirrer is preferably, but notlimited, of the type drawn in FIG. 4 a or 4 b. Said aerated slurry beingdischarged across the width of the device, it does not need to be spreadagain over the width of the facing material. Thus, in contrast with theexisting art in which foamed slurry are poured at discrete locations, abubble size gradient can then be avoided by a continuous and consistentflow of aerated slurry onto the facer. The aerated slurry will exit theblender flowing over the wall 67 and will then be in contact with thefacing material 68. Preferably a separation wall 69 is placedsubstantially in the central portion of the mixer and close to thestirrer, in order to clean up the stirrer if needed and to ensure thatonly aerated material is deposited on the facing material. The blenderrotates in counter-clockwise working like a pump to move the aeratedslurry to the facer. Typical dimensions for a 20 m3/h flow rate ofprimary slurry are diameter of the rounded part about 250 mm and a widthof about 1200 mm.

FIG. 8 discloses a further variant of the second mixer used in theinvention, used in lab scale development. It comprises a barrel 70, witha tee 71 at its bottom for receiving the slurry (which may bemanufactured according to any high-shear process) through pipe 72 andair through pipe 73. Air and the slurry mixes to some extent in the tee,and then the mixture penetrates into barrel 70. Barrel 70 is equippedwith a rotating shaft with agitator blades 74 a, 74 b, etc., e.g. 8blades par stage, where the shaft would comprise e.g. 4 stages, with thelower stage being close to the inlet into barrel 70. Barrel 70 will showan inclined top discharge 75. For example, the barrel might be of about90 mm inner diameter, with blades of about 40 mm radius and 1 mm thick.The barrel will be about 210 mm high up to the lowest part of thedischarge inclined part 75, and the blades will be along the shaftseparated by about 60 mm each. The inlet of the tee inside the barrelhas a diameter of about 15 mm.

Use of a nozzle for injecting air is beneficial for some embodiments ofthe invention. Expansion of air in the slurry after injection,especially by the nozzle, is some aspects beneficial for airdistribution. Also, the nozzle makes the design simpler and would beless prone to packing off with set gypsum and more tolerant of thefibers, if and when used.

The high-shear mixer used in the invention is typically one in which theperipheral speed is generally at least 400 m/min, preferably from 500 to700 m/min and an average residence time of 1 to 10 seconds in order tocreate homogeneous and lump-free slurry.

The secondary blender is generally characterized by the capability todistribute the air appropriately through the slurry (this blender ormixer cannot generally be characterized by shear or speed alone). Theoperative conditions depend upon the basic design of the mixer, themeans of introducing air, the viscosity of the slurry, the averageresidence time and the desired air bubble size distribution. The skilledman would know how to adapt dimensions and rotation speeds by routinetests, so that the final operative conditions will ensure a goodblending of bubbles into the slurry. If the air is already introduced infinely divided bubbles a gentle blending to homogenize the blend isgenerally sufficient. In the case where the air is introduced in largerbubbles or as a continuous stream the mixer should be able to grind downthe bubble size, if it is so required. In a horizontal tube mixer of thetype shown in FIG. 4 or a vertical mixer as shown in FIGS. 5 and 6,having a whisk type agitator, the operation mode can by described by thespeed of the wires and the product of number of wires times averageresidence time. The values are then determined after routine testing.

The invention also provides a process for manufacturing wallboardshaving layers and/or edges of densities higher than the core. It isknown to produce hard edges by applying specific flows of gypsum slurryat the time the slurry is cast onto the moving band. In embodiments ofthe invention, part of the slurry produced by the high-shear mixer,which is not foamed or foamed to a rather low extent, is diverted andused as the flow for the hard edges. Similarly, part of the slurryproduced by the high-shear mixer can be used for producing the denselayers which are present between the foamed core and the facer. In thatrespect, one may revert to U.S. Pat. Nos. 5,085,929 and 5,116,671 toBruce, hereby incorporated by reference. It is also within the ambit ofthe invention to use a small amount of prefabricated foam in the firstmixer, for example to have edges of given density (for example ifnon-foamed slurry would result in a too hard edge). The amount ofprefabricated foam introduced will depend on the final propertiesdesired.

The broad bubble distribution can also be achieved by more than one airblender with each one forms a part of the distribution. Thosedistributions are then recombined to form the desired distribution.

The resulting cellular cementitious slurry comprises bubbles of varioussizes. Present embodiments of the present invention allows for acompromise to reconcile the size of the bubbles and their tendency tosegregate in dense slurry. It may be possible to obtain a bimodaldistribution of bubble size in the secondary blender. It iscontemplated, albeit not preferred, to inject a small volume of foam inthe first high-shear mixer in order to create slurry lightened by verysmall bubbles. In such a case, the direct air injection in the secondmixer would be configured to create larger bubbles and hence a bi-modaldistribution of the desired balance.

The cementitious material can be any material that will set with water.Preferably the cementitious material is plaster, i.e. hydratable calciumsulfate (anhydrite or α- or β-hemi-hydrate). It may also be any knownhydraulic binder. Cementitious material is generally a fine-grainedpowder with a median particle size in the range of 5 to 100 μm. Specificembodiments of the invention are particularly designed for quick-settingcement, having a set time less than 30 min, preferably less than 20 min,more preferably less than 10 min.

The material may also comprise aggregates and/or fillers. Aggregates areinert particles with a median size essentially higher than the cement.Fillers are inert powders with a median size essentially lower than thecement. Examples of fillers are fumed silica, fly ash, blast furnaceslag, micro-silica and fine limestone. Examples of likely aggregates arelightweight vermiculite, perlite, micro-spheres, and expanded shale,while heavy aggregates would be silica or limestone sand.

The foaming agent that may be used can be, but is not limited to, anyone that is used in the art of plasterboard e.g. an alkyl-ether-sulfateand/or alkyl-sulfate. Examples can be found in the followingpublications, which are incorporated by reference. U.S. Pat. No.4,676,835, U.S. Pat. No. 5,158,612, U.S. Pat. No. 5,240,639, U.S. Pat.No. 5,085,929, U.S. Pat. No. 5,643,510, WO-A-9516515, WO-A-9723337,WO-A-0270427 and WO-A-0224595. The amount of foamer used is classicaland can be from 0.01 to 1 g/l of slurry (expressed on active materialover solid content of the slurry.).

In one embodiment, the slurry and the resulting set cementitiousmaterial will comprise fibers. The amount of fibers is typically from0.05 to 5% by volume, based on the volume of the primary slurry. Theyare typically 3 to 20 mm long and have typically a diameter of 10 to 20μm. Glass fibers or high modulus synthetic fibers are suitable. In otherembodiments, the invention is practiced in the absence of fibers. In theabsence of fiber means that the amount is less than 0.01% by weight,preferably less than 0.001% (only unintended impurities) and typicallyno fiber will be present at all. A fiber is any fiber typically used inthe art. One can refer to U.S. Pat. No. 6,443,258, which is incorporatedby reference herein. “In the absence of fibers” does not exclude thepresence of cellulosic material, especially originating from reclaimmaterial, as is typically used in the instant field.

The resulting set cementitious material can have a void volume that canvary within broad limits. The void space of a hardened cementitiousmaterial consists of two classes: the voids left by evaporated water andthe bubbles created by air. Generally, the volume of the water voidsdepends only on the water/plaster ratio, whereas the aerating processcan control the air bubble volume. For a plaster, the water voids varyfrom about 40 to about 65% vol or W/P ratio of 0.45 to 1.05respectively. The air bubble volume for a given W/P ratio, say 0.65,ranges from about 25% vol to about 83% vol for densities from 900 to 200kg/m³ respectively. Thus, e.g., for a given density of 400 kg/m³ and aW/P ratio of 0.65 it was found a water void of about 17.5% vol and abubble volume of about 65.5% vol, resulting in a total pore volume ofabout 83% vol. Hence, the total % vol in the set composition may varybetween broad limits; it may range from 47 to 95% vol in one embodimentwhile in another embodiment it ranges from 53 to 75% vol.

Facer materials are those that are used in the art in a conventionalmanner. In one embodiment, the facer is paper. In another embodiment,the facer material is a non-woven mat, preferably a glass mat or a matformed of other fibers (e.g. synthetic fibers or a mixture of cellulosicfibers and synthetic fibers). The use of composite facers with two ofmore layers of different compositions and orientations fibers is alsoencompassed. The cementitious slurry may penetrate partly in the facer,fully, or the facer may even be embedded in the cementitious core.

The resulting board can be a dense board or a lightweight board, withcore densities from 200 to 1100 kg/m³.

It should be understood that any additive classically used in the artcould also be used in the instant process. The additives are thoseinfluencing the behavior of the slurry like retarders/accelerators butnot limited to them and additives influencing the behavior of the finalproduct like water repellents and biocides but not limited to them. Therange of additives is very wide as will appreciate the skilled man.

Resins for the improvement of the mechanical and/or aestheticproperties, known in the art can be added. Examples of resins beneficialalone or in combinations are: polyacrylic, polystyrene, polyvinylchloride, polyolefin, polyurethane, cellulosic, polyalcohol, polyamide,polyester, polyether, polyphenolic, polysulphide, polysulphone,silicone, fluoropolymer. These types of resins can by combined inco-polymers or other combinations, e.g. as styrene-butadiene copolymers.

Examples of couples retarder/accelerator are conventional plasterretarder/BMA, sodium polyacrylate/aluminum sulfate and sodiumphosphonate/zinc sulfate.

A bubble stabilizing agent can also be used.

A water-soluble viscosity modifier can also be used. Examples arepolymers (cellulosic, polyalcohol, polyurethane, polyester, polyether,polyacrylic, co- and terpolymers thereof), clay (modified/natural),fumed silica, hydrophobically-modified or surface-modified additives.

In the conventional plasterboard production the slurry coming out of themixer has the tendency to be stiff compared with discrete laboratorysamples of the same water/plaster ratio. This phenomenon has to do withthe forced setting acceleration and the fact that a conventionalcontinuous plasterboard mixer may retain parts of the slurry much longerthen the average residence time. Thus, in the art it is well known tocombine retarders and accelerators or in order to postpone the firststiffening as close as possible to the forming moment.

The two step mixing process of embodiments of the invention allows oneto enhance the desired effect by separating time and location of theaddition of the two additives, where the retarder would be added in thefirst step and the accelerator in the second step.

Furthermore, it has been found that the addition into the first mixer ofa product blocking more or less completely the rehydration combined withthe addition into the second mixer of a product which neutralizes theblocking agent, is favorable for the process. Such a couple of additivesis sodium polyacrylate (e.g. of a molecular weight of about 2000) asblocking agent and an aluminum salt as e.g. aluminum sulfate as theneutralizer. For the purposes of the present disclosure theblocking/unblocking (neutralizer) agent will be considered asretarder/accelerator. The accelerator is typically added just at theinlet of the second mixer.

One embodiment comprises the steps of first preparing a slurry ofplaster and water (optionally with additives) but without foaming agent.The foamer is then added after the slurry is prepared; typically thefoamer would be injected just at the inlet of the second mixer (i.e. atthe same time the accelerator is added to the second mixer). Not addingthe foamer initially may also further reduce air entrainment in thefirst mixer, if any.

In another preferred embodiment, the foamer is added together with theaccelerator. This provides additional benefits, since the acceleratorefficiency appears improved. Also, the pour time of slurries with foamerin the gauging water is lower than with foamer added together with theaccelerator.

The embodiment with foamer added after the first mixer is especiallyuseful for applications to standard wallboards lines, where thehigh-shear mixer (typically a pin mixer) will serve as the first mixerof the invention, without any substantial risk of air entrainment. Thematrix obtained in this embodiment is supposed to be stronger.

Exemplary amounts of additives are 0.1 to 5 weight percent.

For the measure of slump, one will use the Schmidt ring. The NF B 12-401or ISO DIN 3050 standard (Schmidt ring: internal diameter 60 mm, height50 mm) is applied. After sprinkling the plaster into water for 15 secand letting it soak for 30 s, the mixture is stirred for 30 s beforefilling the Schmidt ring. The ring is removed at 1 min 15 s and thediameter of the spread slurry is measured.

EXAMPLES

Samples for comparison in flexural strength were prepared in thefollowing composition:

Weight (php, for 100 parts Component of plaster) Plaster 100 Ball millaccelerator 0.44 Starch 0.48 Potash (dry) 0.1 Retarder liquid 35% active0.014 Plasticizer liquid 40% active 0.27 Foaming agent liquid 40% active0.16 Water 70

The method used for manufacturing conventional formulations in thelaboratory is as follows. Weigh the dry components together except thepotash and dry blend. Foaming agent is blended in 30% of the gaugingwater and foamed for 60 seconds in a Waring® blender. Weigh 70% of thewater the plasticizer, retarder and potash, blended together and keptapart from the foam. Dry components are placed in a Hobart® rotatingorbital mixer in a bowel with a wire whisk mixing attachment. Water andadditives are poured on the dry materials and agitated for 5 seconds onspeed 2. The mixer is stopped and the speed changed to speed 3.Prefabricated foam is added to the mixing bowel and mixed on speed 3 for5 seconds. The mixer is stopped and the slurry is poured into anenvelope of wallboard cardboard. The cardboard is supported by verticalwalls spaced at the designed thickness of 12.5 mm. The sample is removedfrom support and trimmed to size at final set. Then it is dried at ahigh initial temperature and low final temperature in a high air flowconditions until dry. The sample is conditioned at 40° C. for 24 hours.Then it is weighed and broken in three point flexure test.

The direct air injection method of embodiments of the invention is thefollowing. Weigh the dry components together except the potash, if any,and dry blend. Weigh the water, the plasticizer, retarder foaming agentand potash, if any, and blend together. Add the dry into the wet. Mixthe components with a high-shear mixer being a kitchen mixer simulatingthe mixer in FIG. 3 for 30 s to a fluid consistency of 220 mm in a fewseconds. Pump the slurry in to the air blender of FIG. 5 at a rate of100 l/h and inject air through a fritted bottom, at a rate of 1000 l/hand catch the discharge. The forming, drying and testing procedure isthen the same as above.

The resulting samples had the following properties:

Property Comparison Invention Thickness (mm)   12.46   12.47 Density(g/cm3)    0.617    0.627 Plastic yield (MPa)    0.98    1.16 Young'sModulus (MPa) 1 374 1 711

What is claimed is:
 1. A continuous process for manufacturing setcellular cement, comprising the steps of: (i) mixing a cementitiousmaterial, water, a foaming agent, a retarder, an accelerator andoptionally additives in a primary mixer under high-shear mixingconditions, an amount of the foaming agent being 0.01 to 1 g/l ofslurry, an average residence time being 1 to 10 seconds, and theperipheral speed being at least 400 m/min, into a free flowing slurryhaving a slump of at least 100 mm, wherein the primary mixer is aninclined disc mixer; subsequently (ii) injecting air by a nozzle in asecondary mixer under controlled-shear mixing conditions, the peripheralspeed being lower than the high-shear mixing conditions of step (i),into the slurry of step (i) and distributing air through the slurry toform a cellular slurry, wherein the secondary mixer comprises a tee forreceiving the slurry through a first pipe and air through a second pipe;subsequently (iii) casting said cellular slurry of step (ii); and (iv)finally allowing said cellular slurry to set, in which the cementitiousmaterial is calcium sulfate α-hemi-hydrate, calcium sulfateβ-hemi-hydrate or a blend thereof, and the final set product has a coredensity between 200 and 1100 kg/m³, wherein a water/plaster ratio of theslurry is 0.65 to 1.05 by weight, the plaster being the cementitiousmaterial.
 2. The process of claim 1, in which step (i) is performed inthe absence of fiber.
 3. The process of claim 1, in which step (i) isperformed in the presence of fibers.
 4. The process of claim 1, in whichstep (ii) is performed under low-shear mixing conditions.
 5. The processof claim 1, in which step (i) is carried out in the absence of addedprefabricated foam.
 6. The process of claim 1, in which step (i) iscarried out in the presence of added prefabricated foam.
 7. The processof claim 1, in which the slump of the slurry obtained at step (i), is atleast 150 mm.
 8. The process of claim 1, in which the slump of theslurry obtained at step (i), is from 200 to 250 mm.
 9. The process ofclaim 1, in which, in the final set product, the pores volume created bywater voids ranges from 20 to 65% vol and the cell volume created byinjected air ranges from 3 to 50% vol.
 10. The process of claim 1, inwhich, in the final set product, the total pore volume ranges is between47 and 95% vol.
 11. The process of claim 1, in which, in the final setproduct, the total pore volume ranges from 53 to 75% vol.
 12. Theprocess of claim 1, in which the cement further comprises at least oneaggregate or at least one filler or both.
 13. The process of claim 1, inwhich step (i) further comprises the step of retarding the setting ofcement while step (ii) further comprises the step of accelerating thesetting of cement.
 14. The process of claim 1, which further comprisesretarding the setting of the plaster and accelerating the setting of theplaster.
 15. The process of claim 1, wherein the retarder comprisessodium polyacrylate and the accelerator comprises aluminum sulfate. 16.The process of claim 1, wherein the retarder comprises sodiumphosphonate and the accelerator comprises zinc sulfate.
 17. The processof claim 1, in which step (i) or step (ii) or both further comprises thestep of adding a strength enhancing resin into the slurry.
 18. Theprocess of claim 17, in which the strength enhancing resin is astyrene-butadiene copolymer.
 19. The process of claim 1, in which step(i) or step (ii) or both further comprises the step of adding a bubblestabilizing agent into the slurry.
 20. The process of claim 1, in whichstep (i) or step (ii) or both further comprises the step of adding awater-soluble viscosity modifier into the slurry.
 21. The process ofclaim 1, in which step (i) comprises two sub-steps (a) and (b), wheresub-step (a) comprises the step of mixing the cementitious material,water, the retarder and optionally additives and sub-step (b) comprisesthe step of adding the foaming agent and the accelerator to the slurryof sub-step (a).
 22. The process of claim 21, in which sub-step (a) isperformed under high-shear mixing conditions.
 23. The process of claim1, which comprises, between step (i) and step (ii), a step of spreadingthe slurry of step (i) prior to the introduction of air.
 24. The processof claim 1, in which step (iii) comprises the step of depositing saidslurry on at least one moving facer to form a cellular core.
 25. Theprocess of claim 1, in which step (iii) comprises the step of depositingsaid slurry on at least one moving facer to form a cellular core andwhich further comprises diverting part of the slurry obtained in step(i) as a stream which is deposited to the cellular core.
 26. The processof claim 1, in which step (iii) comprises the step of depositing saidslurry on at least one moving facer to form a cellular core and whichfurther comprises diverting part of the slurry obtained in step (i) as astream which is deposited onto or beneath the cellular core or both. 27.The process of claim 24, in which the moving facer is paper.
 28. Theprocess of claim 24, further comprising the step of removing the facerafter the cement has set.
 29. The process of claim 24, in which themoving facer is a non-woven mat.
 30. The process of claim 24, in whichthe moving facer is a non-woven glass mat.
 31. The process of claim 1,in which step (ii) comprises the sub step of expanding air betweeninjecting and distributing.
 32. The process of claim 1, wherein, in thefinal set product, the total pore volume is about 83% vol.